In the context of controlling the rotation speed of electric motors, a so-called rated speed signal is usually specified to a rotation speed controller, i.e. the rotation speed controller is instructed by means of a suitable signal that the motor is to run at, for example, 32,246 or at 1100.5 rpm.
That rated speed signal can be, for example, a voltage between 0 and 5 V, or a frequency, or the ratio between signal length and period length (also called the pulse duty factor) in the case of a periodically recurring signal as depicted at 68 in FIG. 5. A signal of this kind then has a specific frequency that will usually lie somewhere between 2000 and 5000 Hz, and the magnitude of the pulse duty factor, which can lie between 0% and 100%, indicates to the rotation speed controller the speed at which the motor is to run at that moment; for example, a pulse duty factor of 0% can mean that the motor is stationary, and 100% can mean a high rotation speed.
Microprocessors (μP) or microcontrollers (μC) are often used for the control and regulation of such motors, the term “microprocessor” (μP) being used as a general term for both. A μP of this kind usually controls the commutation of the motor—assuming the latter is an electronically commutated motor (ECM)—and it also serves to regulate the rotation speed and to perform other functions as applicable.
A μP of this kind requires as a target value for its controller a numerical value, e.g. “37” or “214”; in other words, in a signal of the kind described above, the ratio of signal length to period length, or the ratio of signal off-time to period length, must be converted into a suitable numerical value that lies, for example, in the numerical range from 0 to 255 or 0 to 1023. This requires that both the period length and the signal length be sensed as accurately as possible, to the extent possible with an inexpensive μP such as those used for cost reasons in motors.